Magnetic sheet material provided



Patented June 9, 1953 MAGNETIC SHEET MATERIAL PROVIDED WITH A SEPARATORCOATING AND METHOD OF MAKING SAME John C. Robinson, Pittsfield, Mass,assignor to General Electric Company, a corporation of New York NoDrawing. Application March 14, 1951, Serial No. 215,650

7 Claims.

The present invention relates to a separator coating for magnetic sheetmaterials. It is particularly concerned with a refractory coating whichcan be applied to magnetic sheet materials to function as a separatorbetween adjacent laminations of such sheet materials during heattreatment thereof. The term sheet material as used herein and in theappended claims is intended to include both strip material used, forexample, in forming a wound transformer core as well as the cut orpunched laminations employed in forming stacked transformer cores andthe laminated stator or rotor components of motors, generators and thelike. Examples of magnetic sheet materials are nickel iron alloys,silicon steel, common iron, and similar materials used in theabove-identified electrical apparatus.

In accordance with present-day practice, such magnetic sheet materialsare supplied to the conlike, the product is given a final heat treatmentfor the purpose of developing its magnetic properties. This heattreatment is ordinarilya closed or box anneal, generally in a controlledatmosphere. If the material to be heattreated is still in the form of astri of substantial length, the

strip is conveniently wound into the form of a roll and placed in theannealing furnace in that form. If the material is in the form of platesor sheets of reasonable length or in the form of small punchings such asthose employed in motor manufacture, these are stacked or packed intoboxes for loading into the heat treating furnace. In either case, thematerial is heat treated in a multi-ply assembly in which the surface ofadjacent laminations of the magnetic sheet material are in contact withone another over relatively large areas with the result that at the moreele- 'vated temperatures employed for developing the magnetic qualitiesof the material, the adjacent laminations tend to stick together unlesssome means is provided for separatin the laminations during the heattreatment.

Heretofore, to prevent such sticking, a, finelydivided refractorymaterial, such as magnesium oxide, alumina, calcined dolomite, rzirconia, has been carefully dusted on to the surfaces of the magneticsheet material or applied thereto in the form of a slurry in water justprior to the time the material is wound into the form of ,a core or rollor stacked in preparation for the box anneal. Because of the nature ofthe coating material, the manner in which it is applied, and the factthat it is easily dislodged during handling of the material, it wasnecessary that this operation'be carried out immediately prior to theannealing step. Due to the ease with which such coatings could bebrushed off by normal handling of the coated material, it was notpossible to apply the coatings to the magnetic sheet material untilafter all of the preliminary operations such as punching or cutting hadbeen carried out. Even the normal handlin of the wound strip materialswould frequently result in the refractory material being rubbed off ordisplaced from some surface areas to an extent such that sticking wouldtake place during subsequent anneal of the wound core.

To simplify the coating process, it has been proposed that the separatormaterial be applied in the form of a paint comprising the finely-dividedrefractory material suspended in a solution of a decomposable bindersuch as a solution of an alkyd resin, cellulose acetate or the like insuitable organic solvents. After evaporation of the solvent, theresinous component of the paint served to bond the refractory materialto the surfaces of the magnetic sheet material in the form of a filmwhich would not become loose or dislodged during normal handlingoperations, including the punching or cutting of a sheet or strip intosuitable laminations. This process, however, involved on a commercialscale the use of large quantities of expensive binders and solvents. In

.; addition, when such binders were used, the refractory layer remainingafter decomposition of the binder durin the annealing operation was nolonger bonded to the magnetic sheet material. It could be brushed fromthe surfaces with the same ease as those separator coatings applied inthe dry state or from a water slurry. This was apparently due to thefact that the individual particles of refractory oxide as applied to themagnetic sheet material were completely coated or surrounded by a layerof the resinous binder and actually spaced from the surfaces of thesheet material by this layer. Upon decomposition of the binder, theseparation was still existent so that at no time during the process wasthere a substantial percentage of the oxide particles in such intimatecontact with the surfaces of the sheet material as to obtain an actualbonding or adhesion of the particles themselves to the surfaces of thesheet material during anneal to form a permanent film thereon.

It is, therefore, an object of the present invention to provide aninexpensive, general purpose coating which can be easily applied tomagnetic strip material and which will resist brushing off by normalhandling of the strip material either while it is being punched intolaminations or wound into toroidal cores, or being shipped. It

is another object of the invention to provide a.

refractory separator coating which will adhere to the sheet materialprior to andduring. the-necessary annealing operations and which willnot affect the magnetic quality of the sheet material after anneal. Afurther object of the invention is to provide a separatorcoatingcontaininga decomposable binder which bonds the particles ofseparator material in such intimate, contact with the surfaces of thesheet material that upondecomposition of the binder during the heattreatment, the particles of separator material actually adhere to thesheet material to form thereon a permanent inorganic layer havinginsulative qualities.

The refractory component of the coatings of the present inventionincludes any one or more of those finely-divided refractory orsemi-refractory materials previously-employed for this purpose. Examplesof such materials are magnesia, alumina, calcined dolomite, zirconia,silica, or mixtures of two or more of these oxides.

The above objects and others which will become apparent from thefollowing description of the invention have been attained by providing acoating for magnetic sheet materials comprising a water-soluble,colloidal cellulose ether, such as methyl celluose, as theheat-decomposable binder for the refractory separator material. It hasbeen found that coatings of the methyl cellulose bonded refractorymaterial tightly adhere to the surfaces of the said material so that thecoated sheets may be shipped, handled, punched, or

wound through tension devices without seriously affecting the adhesionof coating. Following anneal of the coated material in any of theatmospheres previously employed in the anneal of such magneticmaterials, there is no impairment of the magnetic characteristicsthereof and no sign of adhesion between laminations during the heattreatment.

Methyl cellulose, which is the preferred watersoluble cellulose ether,is a material obtainable commercially in the form of a dry powder whichcan be dispersed in water to obtain what appears to be a colloidalsolution. Such a colloidalsolution is an excellent base for preparingasuitable slurry or suspension of'the refractory oxide. It has been foundthat the amount of the methyl cellulose which must be included in'such awater slurry of the refractory oxide to obtain the desired bondingaction between the dried coating and the magnetic sheet material isquite small and ordinarily will not exceed about 2 per cent by weight ofthe slurry. Even in those cases where punchings, for example, in theform of small transformer or motor laminations are. punched from thecoated material, it will be found that the methyl cellulose bondedcoatings are not adversely affected by the punching operations. Toobtain these results, the amount of methyl cellulose employed shouldcomprise at least about 0.3 per cent to .5 per cent by weight of thetotal slurry. While the binding action of the cellulose ether is such asto permit the usual processing of thecoated material, the cost of acellulose ether coating is substantially less than the. cellulose 4.ployed. In addition, as compared with the cellu-- lose acetate or alkydresin coatings containing the same oxide or mixture of oxides, the driedmethyl cellulose coatings are substantially thinner and hence provide aspace factor advantage during the annealing operation.

Another advantage of the present invention is that the coating can beapplied to the continuous strip material as it comes from the supplier.The coated material can be cut or punched into laminations or strips ofthe desired size and shape by the consumer and the coated productsstacked or rolled into the form of a multi-ply assembly for thefi'nal.anneal without further treatment in so far as the problem of separationof adjacent laminations in the assembly during the anneal is concerned.

In preparing a slurry suitable for the practice of the presentinvention, it is important that hot water, preferably at a temperaturefrom to C. and'in an amount comprising from one-fifth to one-third ofthe total'water required in the final slurry be used to form apreliminary dispersion of the methyl cellulose in the form of acolloidal methyl-oellulcse-water mixture. Upon agitation of the mixtureof hot water and methyl cellulose, as, for example, after five minutesagitation, the methyl cellulose becomes substantially wetted after whichthe mixture is cooled and the remaining water at room temperature orcolder is mixed with the cooled water dispersion of the methyl celluloseto form a stable colloidal dispersion of the methyl cellulose. Theformation of the dispersion of the methyl-cellulose in the total volumeof water will be accelerated as the temperature of the mixture islowered once the methyl cellulose has been wet by the hot water. Themagnesia or other refractory material or materials can then be mixedwith the resultant methyl cellulose dispersion-or if desired therefractory material can be incorporated in the form of a slurry in thecold water which is mixed with the cooled water-methyl cellulosemixture. Preferably, the resultant dispersion of refractory material iscooled to at least room temperature prior to use.

Once the'refractory oxide slurry has been applied in the form of a thincoating on the magnetic sheet material by any suitable means, such as byimmersion, spraying, brushing, etc, the water should be removedtherefrom by drying at temperatures which do not exceed about C. and. ata not too rapid a rate in order that the film forming properties of thedried methyl cellulose'are not'destroyed. If the Water is removed toorapidly or at more elevated temperatures, the binding effect of themethyl cellulose may be destroyed so that the coating is easily brushedfrom the surface of the magnetic material during subsequent handling.

During the anneal of they coated sheet material, most of the refractoryparticles, for example, the magnesia particles become bonded to thesurfaces of the sheet material to form a permanent inorganic. film.Thishas been found to be the case with all of the magnetic sheetmaterials including nickel steels, common iron, etc. In some cases, aswith silicon steel, this bonding may also be of a chemical natureresulting from the re action of the magnesia, calcium oxide or thelik'ewith the constituents, such as silica, present in the. natural scale onthe steel. These thin coatings of a thickness of about 0.1 mil or lesshav e acetate and alkyd resin coatings heretofore em- 75 i su t g p p ts efiective t0 c c y sulate the laminations from one another duringsubsequent use in a transformer or the like.

In accordance with a further modification of the invention, the coatingwhich is applied to the steel may contain two or more reactive oxideswhich at heat treating temperatures will react to form adherentelectrically insulating coatings having better insulating values thanthose obtained with a single oxide. For this purpose, a small amount ofcolloidal silica can be incorporated into a slurry of magnesia or limeprior to the application thereof to the surface of the magnetic sheetmaterial. For example, satisfactory coatings on the annealed producthave been obtained by employing a slurry composed of 0.5 per cent byweight methyl cellulose, 7 per cent magnesium oxide and 2 per centcolloidal silica, balance water. When magnetic sheet materials, such ascommon iron, nickel steels, etc., having such a coating are heat treatedat temperatures in excess of 800 C., there is a reaction between therefractory oxide and the silica to form on the surfaces of the materiala thin insulating coating probably comprising a silicate of therefractory oxide as, for example, magnesium silicate. Similar resultswith the formation of a similar coating can also be obtained by the useof lime or calcined dolomite in place of magnesia.

In general, the slurry as employed in the practice of the presentinvention should preferably contain at least 2 per cent by weight of therefractory oxide. The maximum percentage of the refractory oxide willdepend somewhat upon the ease with which it can be maintained in thedispersed or suspended form in the slurry and the maximum thickness ofthe final coating which can be tolerated. If desired, a second coatingcan be applied after the first has dried. In some cases it may beadvantageous to form the desired coating by a process in which the sheetmaterial is first coated with a dilute methyl-cellulose-water coatingand after this coating has dried the sheet material is passed through awater suspension of the refractory oxide which adheres to the cellulosefilm. After this coating has dried, it will be found that the refractoryoxide is bonded to the sheet material to substantially the same degreeas when the oxide and cellulose are mixed prior to application to thesheet material.

During the anneal of the coated material at temperatures up'to 1200 0.,generally from about 700 to 1175 C., depending upon the particularmagnetic material involved, and particularly dur-- ing the earlierstages of the anneal when the material is being brought up totemperature, the methyl cellulose disintegrates into substantiallyvolatile components, leaving the finely-divided refractory in intimatecontact with the surface of the magnetic material to which it becomesbonded at the annealing temperatures.

A particular advantage of the present invention is that the magneticmaterial in continuous strip form can be coated with the methylcellulose slurry by the manufacturer of the magnetic material just priorto the winding of the strip material into the form of rolls suitable forshipment to a consumer. This advantage will become more apparent when itis considered that ordinarily unrolling coating and rerolling of thestrip material constitute additional operations on the part of theconsumer whereas the coating process readily fits into those operationssubsequent to the final open anneal and prior to the winding of thestrip material for shipment by the manufacturer thereof.

What I claim as new and desire to secure by Letters Patent of theUnited'States is:

l. The method which comprises providing magnetic sheet material with acoating composed of a suspension of a finely divided refractoryseparator material in water containing from about 0.3 to 2% by weight ofmethyl cellulose, drying the coating, at such a temperature and such arate that the film forming properties of the methyl cellulose are notdestroyed, mechanically processing the coated material to place it inthe physical condition desired prior to the final anneal thereof todevelop its optimum magnetic properties, and box annealing the coatedsheet material in multi-ply assembly to develop the magnetic prepertiesthereof and to form on said sheet material a tightly adhering coating ofthe inorganic separator material.

2. The method for providing a magnetic sheet material with an adherentcoating of finely divided refractory material adapted to act as aseparator during the box anneal of the magnetic sheet material, whichmethod comprises applying the refractory material to the surfaces of themagnetic material in the form of a suspension thereof in a methylcellulose-water mixture containing from about 0.3 to 2% by weight methylcellulose and immediately drying the coating at such a temperature andsuch a rate that the film forming properties of the methyl cellulose arenot destroyed to form a tightly adherent coating of the refractorymaterial bonded to the magnetic material by means of the methylcellulose.

3. The method which comprises providing a continuous strip of magneticmaterial with a coating of a finely divided refractory material in watercontaining from about 0.3 to 2% by weight of methyl cellulose as adispersing and-bonding agent, drying the coated material at such atemperature and such a rate that the film forming properties of themethyl cellulose are not destroyed and subjecting the coated material toa box anneal to decompose the methyl cellulose binder into substantiallycompletely volatile components leaving a layer of the refractorypowderadhering to and separating the annealed laminations.

4. The method which comprises providing a continuous strip of magneticmaterial with a coating of slurry of a finely divided refractory oxide,water, and from 0.3 to 1% by weight methyl cellulose, drying the coatingat such a temperature and such a rate that the film forming propertiesof the methyl cellulose are not destroyed to remove the water contentthereof and subsequently box annealing the coated material to decomposethe methyl cellulose component thereof and to leave on the surface ofthe magnetic material a tightly adhering coating of the refractoryoxide.

5. The method for providing magnetic sheet material with an insulatingcoating during the anneal thereof, which method comprises applying tothe material prior to anneal a coating of a slurry of finely dividedmagnesium oxide, colloidal silica and from about 0.3 to 2% by weight ofmethyl cellulose in water, drying said coating at such a temperature andsuch a rate that the film forming properties of the methyl cellulose arenot destroyed and subsequently box annealing the coated material to atemperature up to 1200 C. to decompose the methyl cellulose andatom-@155 to. form. an: adherent insulatingv coating-of the magnesiumoxide and silica on" the surface of said sheet material.

6. A magnetic sheet material having thereon a thin coating consisting ofat least one finely divided refractory oxide and a small amount ofmethyl cellulose binder bonding the refractory oxide to said sheetmaterial in such intimate contact with the surface of said sheetmaterial that upon decomposition of the methyl cellulose during annealof the coated sheet material particles of the refractory oxide form onthe surface of the sheet material a permanent, inorganic, electricallyinsulating layer.

7. A silicon steel sheet material having there,- on an adherent coatingconsisting essentially of a refractory oxide mixture consisting ofmagnesia and silica in finely divided form and a small amount of;methyl. cellulose; binder "bonding the refractory oxide mixture in vsuchintimate: C011? tactwith the surface of said silicon steel Sheetmaterial that upon decomposition of the methyl cellulose binder duringanneal of the sheet material particles. of the, refractory oxide mixtureform on the surface of the sheet material a permanent, inorganic,electrically insulating layer.

JOHN C. ROBINSON.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,316,745 Robertson Apr. 13, 1943 2,515,788 MOIlill' July 18,1950

1. THE METHOD WHICH COMPRISES PROVIDING MAGNETIC SHEET MATERIAL WITH ACOATING COMPOSED OF A SUSPENSION OF A FINELY DIVIDED REFRACTORYSEPARATOR MATERIAL IN WATER CONTAINING FROM ABOUT 0.3 TO 2, BY WEIGHT OFMETHYL CELLULOSE, DRYING THE COATING, AT SUCH A TEMPERATURE AND SUCH ARATE THAT THE FILM FORMING PROPERTIES OF THE METHYL CELLULOSE ARE NOTDESTROYED, MECHANICALLY PROCESSING THE COATED MATERIAL TO PLACE IT INTHE PHYSICAL CONDITION DESIRED PRIOR TO THE FINAL ANNEAL THEREOF TODEVELOP ITS OPTIMUM MAGNETIC PROPERTIES, AND BOX ANNEALING THE COATEDSHEET MATERIAL IN MULTI-PLY ASSEMBLY TO FORM ON SAID SHEET MATERIAL ATIGHTLY ADHERING COATING OF THE INORGANIC SEPARATOR MATERIAL.